draw-legend indicates whether
a legend is drawn in a chart-or-plot. The legend is a table at the
bottom of the chart with a descriptive string for each chart object
and a visual indication of which graph line or set of bars represent
it. The chart-or-plot will automatically make room for the legend
whenever it is drawn.

The value may be t to draw the
legend, nil to not draw it, or
:when-multiple-objects to draw the legend if and
only if there are multiple objects in the chart-objects property. The default value is
:when-multiple-objects, since a legend is typically
needed only to differentiate multiple chart objects.